Neurons in several areas of the rat and chinchilla brain encode the current direction that the animal's head faces. The activity of these "head direction" (HD) cells is abolished following peripheral vestibular lesions, but the circuitry responsible for generating and maintaining the HD signal has not been established. The purpose of this proposal is to better understand the relationship between the vestibular system and HD cell activity. Specific Aim 1 tests the hypothesis that horizontal semicircular canals in the chinchilla are necessary for normal HD cell activity. If the horizontal semicircular canals are essential for generation of the HD signal, then plugging those canals will disrupt the HD signal. Specific Aim 2 tests the hypothesis that neural activity in the rostral nucleus prepositus hypoglossi (NPH) includes signals that could support HD cell activity found downstream. The rostral NPH receives input from vestibular nuclei and projects to dorsal tegmental nucleus (DTN), where angular head velocity signals have been found. Specific Aim 3 tests the hypothesis that cells in NPH that project to the DTN are distinct from NPH cells that provide input to the abducens nucleus. This will establish whether NPH eye movement signals could contribute to the HD signal. [unreadable] [unreadable] [unreadable]